Many communication channels are noisy; this channel noise is added to intended signals and transmitted to a receiver. Further, many communications devices, including cell phones, are used in noisy environments such as crowds, cars, stores, and other places where background music or noise exists; background noises are often picked up by microphones and are effectively added to the intended voice signal and, unless suppressed at the transmitting device, are transmitted to the receiver.
When either or both channel noise or background noise reaches a receiver, this noise can impair intelligibility of intended voice signals unless a noise suppressor is used.
A typical communications system 200 in which an audio noise suppressor may be used is illustrated in FIG. 2. Audio from a human speaker 202 and background noise sources 204 are picked up by a microphone 206, audio from microphone 206 may be processed by a noise suppressor 208 before being transmitted by transmitter 210 into channel 212. Channel noise may be injected into channel 212 by channel noise sources 214, where channel noise may add to a transmitted signal and received by receiver 216 to provide a noisy signal that may be processed by noise suppressor 218 before driving a speaker 220 and being presented to a listener 222.
A conventional noise suppressor 100 (FIG. 1), useable as noise suppressor 208 at the transmitter end of channel 212 or as noise suppressor 218 at the receiver end of channel 212, receives an audio input 102 into a frequency-domain conversion unit 104. Frequency domain signals are divided into separate signals 108 each representing a frequency band of multiple frequency bands by band extractor 106; these separate frequency band signals are provided to a speech detector 110 that determines from the separate frequency band signals if speech is present in the incoming audio. Each frequency band signal is processed further by a separate per-band unit 112 having a noise estimator 114 and signal-to-noise ratio estimator 116 that provides an estimated signal-to-noise ratio 118 to a gain calculator 120. Gain calculator 120 provides a band-specific gain 122 to a variable gain unit 124 that applies band-specific gain 122 to the separate signals 108 representing that frequency band to provide a band-specific gain-adjusted signal 126. The band-specific gain-adjusted signals 126 are collected by a recombiner 128 and converted by an analog or time domain convertor 130 to either an analog domain or a digital time domain audio output signal 132.
Many variations of suppressors derived from the basic suppressor of FIG. 1. These variant noise suppressors often differ in the SNR estimator 116 and gain calculator 120 subsystems. For example, filtering or smoothing may be added at gain calculator 120 outputs to reduce artifacts by stabilizing gain of variable gain unit 124.
Quality of noise suppression using noise suppressors according to FIG. 1, and related noise suppressors, in systems according to FIG. 2 depends on the quality of noise level estimation in noise estimator 114, because incorrect estimates of noise corrupt the SNR in SNR estimator 116, and thus the determined gain 122 for that frequency band.
There are two types of noise commonly found in noisy audio. A first type of noise is “stationary” noise, such as continuous channel noise or a background noises from constantly running fans, flowing water, or a car engine at a constant distance, where the noise tends to have a fairly constant frequency and amplitude distribution. A second type of noise is “non-stationary,” variable, noise such as background noise produced by multiple moving automobiles in traffic, several people talking while moving through a crowd, barking dogs, television and radio broadcasts, irritated drivers pressing horn buttons, and other non-constant sources. Much background noise picked up by microphone 206 from audio noise sources 204 is non-stationary.
Typical noise suppressors perform much better on stationary than on non-stationary noise, in part because estimation of noise levels in noise estimator 114 is more difficult with non-stationary noise.